Ion mobility spectrometry (IMS) today figures prominently among analytical methods for detection of explosives and is widely used for transport security. This method is highly appreciated for capability to distinguish low quantity of different types of explosives at atmospheric pressure. The main obstacle for sensing some kinds of explosives is their very low vapor pressure so that the limit of detection of state-of-the-art IMS instrumentation 10-13 –10-14 g/cm3 lacks at least an order of magnitude. In this paper we combine promising UV laser radiation along with application of organic compounds (dopants) to improve ionization efficiency and vapor detection capabilities of IMS. Dopants with low ionization energy (toluene and 1-methylnaphtalene) were used for negative ion formation of nitro group-based explosives: trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN). Presence of dopants in the sample results in multiple growth of ion yield at laser intensities lower than 2 × 107 W/cm2. Limits of detection with dopant-assisted laser ionization (4.7 × 10-16 g/cm3 for RDX and 9.8 × 10-15 g/cm3 for PETN) show up to 2-fold improvement compared with no dopant case. Results propose a way to further improve sensitivity of detectors and reduce manufacturing costs by lowering requirements to laser pulse energy and using cheaper lasers.
In this paper we study the influence of the carrier and drift gas composition on ionization processes taking place inside drift chamber of field asymmetric ion mobility spectrometer with laser ionization. Solid state nanosecond laser of YAG:Nd 3+ type with fourth harmonic unit (λ = 266 nm, τpulse = 6 ns, E pulse = 700 – 2500 μJ, ν = 10 – 20 Hz) was used for negative ion generation. In this study we experimentally discover the features of laser ionization of four nitro-compounds: cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT) explosives.
Drift and sample carrier gas were prepared by mixing purified air with different amounts of water vapor and organic dopants. Ion mobility increments were calculated after calibration of field asymmetric ion mobility spectrometer (FAIMS) based on published data for TNT and Iodine and measured alternating separation field waveform. The experimental setup also included drift time ion mobility spectrometer (IMS) which was used to verify linear ion mobility spectra to supplement ion mobility increment values, obtained by FAIMS.
Previous studies of laser ionization with optimization of intensity and pulse repetition rates gave LOD values well below 10−15 g/cm3: 3 × 10−15 g/cm3 for RDX, 8 × 10−15 g/cm3 for PETN and less than 3 × 10−15 g/cm3 for HMX. Common ideas about ionization mechanisms of nitro-based explosives propose that indirect processes with ion-molecular reactions substantially contribute to negative ion formation as well as resonant enhanced multi photon ionization (REMPI) direct processes. Ionization process starts with electron generation by organic impurities in atmospheric air. These organic compounds have low ionization energy and require less than two photons to ionize.
Current research involves doping air sample with such substances as: toluene acetone, naphthalene and chloroform at different UV irradiation modes. Such compounds can act as electron source for rising TNT and RDX ion signal levels above background. Such selectivity enhancement can be a step on the way to achieving even lower detection limits to sense trace explosive vapor concentrations in real conditions.
Aerosol samplers with a recirculating liquid film are promising devices for remote biological monitoring. The presence of the liquid film provides a high survival rate for biological objects. The relatively simpler design allows portability to the sampler, which will make it possible to conduct tests outside the laboratory. In this study an analytical expression, describing the capturing efficiency of aerosol particles in the water film, taking into account the friction forces arising from interaction of water and air in a cyclone-based aerosol collector, was obtained. A new element, took over from the theory of centrifugal sprayers - a vortex chamber, was added to the theory and design of the collector. It allows increasing the initial angular moment of the elements of air volume entering the collector, which leads in appropriately an increase of maximal height of rising liquid film and particle capturing efficiency. To analyze the obtained expressions, graphs of particle capturing efficiency on basic parameters of modified cyclone collector and volumetric air flow were calculated. The graphs made it possible to determine the optimal geometric parameters for the portable cyclone-based collector. The introduced dependence on viscosity made it possible to estimate more accurately the efficiency of the device at various temperatures (including negative temperatures). For the selected parameters, graphs particle capturing efficiency were plotted. Water-alcohol solution and Novec 1230 fluid were used as fluids capable of operating at subzero temperatures. To check the operability of the sampler, tests were carried out to collect samples of sprayed inactivated adenovirus in a microbiological safety box at the Gamaleya Institute. The results of tests are discussed.
The issue of civil security and prevention of terrorist attacks in public places is becoming more and more actual every year. In this regard, increased attention is paid to detection of explosives. Of particular interest are methods to detect trinitrotoluene (TNT), hexogen (RDX), penthrite (PETN), octogen (HMX). Recently, gas-analytical, nuclear-physical, electromagnetic, terahertz, and biological detection methods have been developed. The lowest detection limit was achieved using gas-analytical methods, namely the non-linear ion mobility spectrometry method, with a limit of detection of 5.10-15 g/cm3. However, the question of feasibility of using these methods in real conditions is increasingly raised. There is an opinion that it is much more effective to detect explosives by traces than by vapor. In this work we investigated the possibility of detecting vapors of pure explosives with low saturated pressure of vapors. By the example of pure and faсtory hexogen, using the method of thermal-programmed desorption and mass spectrometry, it was shown that it is hexogen vapor, and not technological impurities or additives with saturated vapor pressure exceeding the saturated vapor pressure of hexogen, that are registered in the gas phase by ion mobility spectrometry. A technique was developed and proposed to concentrate RDX vapors. Using temperature-programmed desorption, the minimal time of concentration and surface fill factor were determined.
Cyclotrhylmethylentrinitramine (RDX) is one of the most dangerous explosive substances. The presence of impurities in this explosive may be important for its determination as residual traces at the scene of incidents, as well as in the quick analysis of passengers and luggage by ion mobility spectrometers. In the work industrially manufactured RDX samples, that were not undergone any purification, were investigated by gas chromatography-mass spectrometry method. The attention was paid to search precursors and possible technological admixtures. On the basis of received chromatograms and mass spectra it was established the presence in the analyzed RDX samples of urotropin, acetic anhydride, acetic acid, triazine and octogen. The mass content of the substances was estimated. Special attention was paid to the products of RDX biodegradation, which can occur both in aerobic and anaerobic environments during its storage. The quantitative content of RDX biodegradation products in samples under study was estimated. Sorption of chemically pure and industrially manufactured RDX vapors on a concentrator from metal meshes were carried out. By the method of thermodesorption mass spectrometry the composition and differences of the concentrated samples were analyzed. Recommendations for taking into account the results obtained when working with vapors and traces of RDX by ion mobility spectrometers were given.
Early detection of pathogens is crucial for ensuring safe living conditions. Conventional sampling methods do not ensure the sustainability of captured microorganisms. Cyclone-based liquid absorptive samplers are the most promising devices in this regard. Most samplers have limited application scope as they are intended for laboratories. In the study we developed a special liquid-based sampler for mounting on a drone. A structure and base values of a cyclone collector were determined. The chosen design provides maximum particle capturing efficiency in terms of low air flow rate and small size of the collector. Projected capturing efficiency for particles of the size of 1mсm is approximately 20%, for particles of the size bigger than 2,5mсm is 100%. Mathematical modeling proved the correctness of theoretical considerations. This made it possible to design and fabricate a prototype of a sampler device which also includes a fan, a peristaltic pump, valves, a sensor for liquid levels monitoring, a display for operation mode controlling. Outdoor sampling on a drone has successfully proved performance efficiency at close to zero temperatures. A study on capturing efficiency was carried out by sampling sucrose aerosol with a fluorescein dye in a microbiological safety box. Specific efficiency of our sampler is 1,18 [relative units/(litre/min)], which is over twice s.e. of 0,43 [r.u./(l/min)] of existing laboratory devices. The device provides for the issuance of collected samples on standard immunochemical test strips for the analysis of adenovirus, rotavirus, influenza and other respiratory diseases with a full analysis time of 10 minutes.
The distinctive features of laser ionization source in field asymmetric ion mobility spectrometry method are studied in this paper. A YAG:Nd 3+ nanosecond laser (λ = 266 nm, τpulse = 6 ns) with variable pulse energy E pulse = 700 – 2500 μJ and frequency ν = 10 – 20 Hz was used as a source of laser radiation. Pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT) cyclotrimethylenetrinitramine (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), explosives were investigated with use of field asymmetric ion mobility spectrometry (FAIMS) method. Ion spectra were recorded by with separating fields 8 – 12 kV/cm. Nickel radioactive isotope 63Ni was used as reference ionization source for explosive molecules.
Non-linear ion mobility spectra of each substance ionized with UV laser radiation and radioactive 63Ni were compared and reasons for similarities and distinctive features are discussed. For peaks of explosives and reactant-ion peaks the dependences of their positions along compensating voltage axis on the magnitude of separating voltage (separating electric field) were measured for all the ion sources and substances.
All the experiments were carried out under controlled ambient temperature and relative humidity (t=25℃, RH 30%). Humidity was supported the same inside and outside the gas system to minimize the influence of water cluster formation on the results obtained. Explosives vapors generators were made with 10mg samples of chemically pure explosives. RDX, HMX and PETN were heated (RDX 50℃, HMX 50℃, PETN 45℃) to increase the concentration of ions.
It was shown that the behavior of the peaks of the explosives at laser excitation is different from the behavior at radioactive source for each of the substances. This indicates the presence of an additional ionization mechanism under laser radiation along with the traditional one. Spectra of reactant ion peaks under radioactive ionization also show difference in ion formation for each substance. Behavior of reactant-ion peaks of each of the substances with laser ion source shows nearly perfect coincidence. This fact can demonstrate well controlled experimental conditions and further confirms the difference in ionization mechanisms for laser and nickel radioactive ion source.
The features of laser ionization of trinitrotoluene and hexogen at registration of ionization products by field asymmetric ion mobility spectrometer were investigated in this paper. The optimal range of intensity to obtain the maximum ion current was determined, and it was shown that composition of ions of explosives is changed with the change of intensity. The values of intensity and radiation at which it is possible to obtain spectra of nonlinear ion mobility with different composition of RDX and TNT ions were determined, which is promising for increasing the selectivity of registration. It was shown that ion peaks of TNT and RDX at laser ionization are characterized by low nonlinear mobility coefficients in comparison with a radioactive source based on Ni63, which indicates a more selective mechanism of laser ionization of these explosives at atmospheric pressure.
The possibility of optimizing the parameters of laser radiation for increasing the efficiency of ionization of vapors of explosives in a laser field asymmetric ion mobility spectrometer (FAIMS) was investigated. The dependences of TNT ion signal on intensity of laser radiation, pulse energy and repetition frequency of laser pulses were obtained. Optimum value of the intensity of laser radiation for ionization in air was experimentally obtained - 3•107 W/cm2. It is shown that defocusing of laser radiation and expansion of the ionization region increase the TNT signal if the radiation intensity remains at the optimum level. It is shown that repetition rate should be such that the interval between pulses is shorter than the time of diffusion of non-ionized molecules into the laser irradiation region.
A scheme for a multi-pass laser beam through the ion source of a spectrometer was proposed. The scheme will effectively use laser radiation and improve the ionization efficiency of nitro aromatic molecules.
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